watchOS 5.1.3 (build 16S5531a) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-12-19T21:06:23Z. The update package is 189.55 MB. This is a delta (partial) update that requires 5.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 8ba0282471f16c2067aea43c7582ddbae6db0862.zip.